Very pleased to be at the Top Analytica Microscopy and Microanalysis Workshop in Turku, Finland where Alemnis AG has launched its new Alemnis Compact Assembly (ACA) system which fits into Hitachi TM3030 and TM4000 Desktop SEMs. Here on the Spectral AB booth with Mats Eriksson and Sasha Vuckovic, new Nordic distributors of Alemnis…

Nanoindentation is a ubiquitous analytical technique used to investigate a wide range of material properties. With increasingly compact geometries and high-resolution, nanoindenters can explore the mechanical properties of microscale samples to obtain results that are analogous to the macroscale, bulk material. The cost and convenience benefits of this are self-evident.

Nanoindenters can be leveraged in a choice of materials science test methods and applications. They are also increasingly pivotal in preliminary testing that enables further, more detailed mechanical analyses downstream.

Elastic modulus, for instance, is a key property underlying a range of dynamic behavioral mechanics including fracture toughness which is commonly calculated according to the Dukino and Swain equation. In brief, this considers fracture toughness a function of an applied indentation load, average crack length, the ratio of hardness to elastic modulus, and a geometric constant.

In this blog post, Alemnis will briefly explore how nanoindenters can be used to determine the elastic modulus of materials.

What is Elastic Modulus?

First, let us offer a short definition of elastic modulus. Often referred to as Young’s modulus, after British scientist Thomas Young, the elastic modulus of a material is a measure of its ability to resist bending or compressive forces and is consequently connected to both hardness and stiffness. Elastic moduli are determined as the ratio of tensile stress to strain within the material’s elastic limits for compressive and tensile loads; essentially comprising a measure of the force required to cause reversible (elastic) deformation. 

Nanoindenters are used to probe materials and measure malleability/brittleness in relation to linear elasticity on a stress-strain graph. This is critical for gaining a thorough understanding of material performance under pre-defined use conditions and forecasting various dynamic deformation mechanics or modes of failure.

How Do Nanoindenters Determine Elastic Modulus?

To measure the elastic moduli of materials, nanoindenters bring a probe tip into contact with the surface at minuscule loads, typically just a few micronewtons (µN). This load is ramped up at user-defined rates to a maximum point and then decreased back down to zero at the same rate. Conventional indentation tests acquire load-displacement curves that are indicative of various mechanical properties. By observing the unloading curve, analysts can observe the elastic recovery of sample materials, which is a fundamental parameter in calculating elastic modulus.

Alemnis Nanoindenters

At Alemnis, we have developed a class-leading nanoindentation unit that pushes the frontiers of nanomechanical testing. The Alemnis Standard Assembly (ASA) exceeds traditional nanoindenters in terms of measurement accuracy and experiment flexibility. If you would like more information about measuring the elastic moduli of your materials with our innovative nanoindenter, simply contact a member of the team today.

The high strain rate behavior of materials is of enormous interest to product developers and the wider research community. Strain rate testing intersects an extremely wide range of market segments and is relevant to a variety of application areas, providing unique insights into how ceramics, electronics arrays, polymers, and so on, can withstand severe loading events.

At the macroscale, there are numerous proven methodologies for ultra-high strain rate testing (impact experiments, Kolsky pressure bar, etc.). However, the micro- and nanoscales remain comparatively unexplored due to the inherent limitations of the established testing methods. One of the foremost techniques used to explore the mechanical properties of sample materials is nanoindentation. Though incredibly valuable, nanoindentation tests typically employ quasi-static loading, which limits available strain rates to fairly low levels (<10^-2s^-1).

Read More: A Guide to High Strain Rate Testing

How to Achieve Ultra-High Strain Rates via Nanoindentation

The problem with high strain rate testing of materials via nanoindentation is that conventional micromechanical indenters with strain gage sensors, or load cells, have low resonant frequencies and poor sampling rates. This has the tandem effect of increased signal noise and low resolution due to slow feedback loops. Resolving load-displacement data at strain rates above 10^-2s^-1 requires a high data acquisition rate and extremely precise actuation frequencies. Only piezoelectric actuation and sensing offer that level of control for micromechanical testing.

Additionally, ultra-high strain rate testing via nanoindentation is facilitated by novel test methods like micropillar compression. Using a piezo-based nanoindenter and a microscale pillar of lithographically-generated or extracted sample material, it is possible to achieve strain rates up to 10,000 s^-1. Owing to the sample’s extremely small dimensions relative to various test parameters (actuation velocity, inertia, wave travel time, etc.), fatigue, impact, and dynamic deformation phenomena can be acquired at ultra-high strain rates without interference.

Currently, fused silica micropillars exist as a proof of concept for how ultra-high strain rate testing can be carried out for numerous other materials with unexplored high-strain rate properties.

Read our full, in-depth article on Ultra-High Strain Rate Testing with a case study on fused silica micropillar compression for more details.

Ultra-High Strain Rate Testing with Alemnis

Alemnis specializes in micro- and nanoscale indentation testing for a wide range of materials, leveraging our pioneering Alemnis Standard Assembly (ASA) with proprietary high- and ultra-high strain rate modules. For strain rates of 1,000 s^-1 and above

Alemnis offers four distinct configurations:

• VHS-1-1, for impact/fatigue testing at strain rates of up to 1,000 s^-1 using a piezo stack actuator with a normal load range of 1N.
• UHS-1-1, for impact/fatigue testing at ultra-high strain rates of >1,000 s^-1 using class-leading SmarTip actuation with a range of +/- 175V.
• UHS-1-3, for ultra-high strain rate tests with additional tri-axial sensing (normal and lateral force) with a lateral load range of +/- 0.1N.
• UHS-3-3, a complete ultra-high strain rate testing solution with tri-axial sensing and actuation enabling novel nanotribology applications.

Observing how materials respond to applied stress in their native state is fundamental to understanding their mechanical properties (creep deformation, fracture toughness, hardness, elastic modulus, etc.). Limited insights can be gained from structural and surface analysis of samples before and after a mechanical load has been applied, typically via indentation. However, this fails to yield accurate data regarding the real-time deformation behavior of samples under test.

In situ mechanical testing is the only way to observe the real-time mechanical performance of samples under applied loads. For instance, conducting nanoindentation in a scanning electron microscope (SEM) environment allows researchers to visualize key parameters of their mechanical tests, including the actual deformation mechanics of samples in real-time.

Read More: What Properties Can Be Measured By Nanoindentation?

In this article, Alemnis briefly explores how real-time materials characterization can be actualized using vacuum-compatible nanoindenters and complementary SEM-based imaging. 

In Situ Mechanical Testing: Challenges & Solutions

Indentation is a common mechanical testing method used to explore the physical properties of different sample types (alloys, ceramics, polymers, etc.). The basic principle involves the application of a mechanical load via a diamond probe tip to determine a sample’s resistance to deformation. Typically carried out ex-situ, various indentation methods have been used to investigate the elastic moduli and hardness scales of materials as a function of load-depth curves. However, conventional indentation is unsuitable for in situ mechanical testing.

Because surface observations are made after the fact, all conclusions drawn from load-depth data that pertain to specific deformation mechanics within the material are largely speculative. This complication is compounded by the rise of nanomechanical testing based on sub-micro scale (μm) indentation. In situ mechanical testing subsequently requires a powerful imaging tool that can offer nanoscale observations of sample mechanics in real-time.

With far superior spatial resolution than optical light-based microscopy, SEMs have significantly shifted the goalposts of modern materials characterization. Prior to the onset of SEM, researchers were limited to nominal magnification ranges of up to 1000x due to the limited resolving power of visible light. A focused electron beam offers magnifying capabilities of up to 50,000x which yields extremely high lateral resolution ranging down to just a few nanometres (nm). Yet the limited geometries and vacuum pressures of sample chambers make it impossible to couple SEM imagery with conventional nanoindenters. 

Small-scale, vacuum compatible nanoindentation modules are subsequently a prerequisite for in situ mechanical testing. This can yield a range of valuable results, providing accurate data regarding crack propagation, delamination, fracture onset, and more.

Real-Time Materials Characterization with Alemnis

Alemnis offers a unique solution for in situ mechanical testing of myriad sample types with real-time imaging via SEM. The Alemnis Standard Assembly (ASA) is a small footprint nanoindenter comprising a piezo actuated displacement head with an integrated sensor for completely closed-loop indentation testing. This module can be mounted and operated within the sample chamber of an SEM with sufficient space, enabling researchers to observe the real-time displacement of their samples down to a single nanometre.

If you would like to learn more about the unique Almenis solution for nanoindentation testing, read our previous blog post: What is a Nanoindenter?

We also provide custom mounting solutions and modifications for SEM integration. Contact a member of the Alemnis team today to learn about costing an ASA configuration with your SEM unit.

A nanoindenter is a versatile instrument used to test the mechanical properties of materials. Primarily developed for localized hardness testing, nanoindentation is now widely used to measure creep, fracture toughness, elastic modulus, stress relaxation, cracking, di slocation nucleation, and the viscoelastic properties of samples. 

In this blog post, Alemnis explores the working principles of nanonindenters in more detail. 

Nanoindenter Working Principles

At the heart of a nanoindenter is a small probe loaded with a calibrated indenter tip, which may be pyramidal, flat, spherical, wedged, or some other shape. This is used to interrogate the surface of a material and measure the subsequent force-displacement data.

Conventional nanoindenters are typically load-controlled instruments where the tip is brought into contact with the surface under a pre-defined load. Once the nanoindenter has contacted the sample, the load is increased and the tip indents into the material. The area of contact between the tip and the sample, the applied force of the nanoindenter, and the depth of displacement are subsequently used to determine the material’s mechanical properties. The disadvantages of a load-controlled system become obvious when doing compression tests on small structures (e.g., micropillars, 3D structures, etc.) where the load feedback loop cannot cope with sudden displacement excursions. In such cases, nanoindentation measurements using displacement control are far superior.

Traditionally, the size and depth of the residual indentation imprint is taken to calculate the material hardness. This is characterized according to one of several indentation hardness scales, including the Vickers and Brinell scales. Nanoindenters have proven valuable for microhardness testing where samples are small or thin, and they have also demonstrated unique performance for measurements where the microstructural properties of a sample are complex or are non-homogenous.

Typical example of a nanoindentation load-depth curve (left) and corresponding residual indent (right)

Applications of Nanoindentation Testing

Nanoindenters are broadly used to test the properties of hard coatings and thin films. As the dimensions of engineering and electronic components continue to decrease, it is vital that local mechanical properties are tested with very little residual impressions. Conventional techniques are largely incapable of testing small-scale components non-destructively, and in best case scenarios may impact the material properties of miniaturized components. Nanoindentation is subsequently used to characterize a range of materials to assist in research and development of new products and solutions. In the case of coatings, a nanoindenter can test the properties of even very thin films (e.g., thickness < 1 µm) without any influence from the properties of the substrate underneath.

Increasingly, nanoindentation has been used to test the complex properties of more dynamic material types including soft, hard, inorganic, and organic samples.

The Alemnis system is fully compatible for in-situ scanning electron microscopy (SEM) experiments, and ex-situ testing with Synchrotron beamlines and standard optical microscopes. This provides new levels of flexibility for dynamic material testing under high strain rates ( up to 10’000 s-1 ) and over very wide temperature ranges (from -150°C up to 1000°C)

Example of micropillar compression experiment with a flat punch diamond indenter

Localized mechanical testing of materials using the Alemnis system can provide new ways of understanding product failure under end-use conditions. It can be used to measure very small volumes of a specific material which can be in the form of a micropillar fabricated by focused ion beam (FIB) milling or by lithographic techniques such as LIGA. Small volume testing can help to understand the fundamental deformation mechanisms and if performed in-situ allows direct observation of the resultant phenomena, e.g. twinning or slip in metals, stress relaxation in ceramics or creep of oxide structures. 

This level of precision has also been explored in the world of emerging biomaterials, offering new understandings of how tissues behave at a cellular level and how this behaviour can be modified. Quantitatively measuring the mechanical properties of biomaterials could provide new insights into biological behaviours, supporting research into innovative pharmaceuticals and medical devices. The Alemnis system can be used to do ex-situ bio-testing in various environments, such as controlled relative humidity, temperature or in liquids (e.g., saline or body-mimicking fluid).

The Unique Alemnis Solution

Alemnis is a specialist in small-scale mechanical testing for materials science research, with a nanoindentation system which can be used in either compression or tension modes. This means that a pillar can be compressed with a flat punch, a surface can be indented with a sharp indenter, or a microscale dogbone sample can be tensile tested, all with the same system. Such high versatility is unmatched and based on years of combined experience.

Read More: What Properties Can Be Measured by Nanoindentation?

The Alemnis system features a true displacement mode as well as various ultra-high strain rate options which permit strain rate jump tests, fatigue testing and dynamic mechanical analysis (DMA). Coupled with the latest state-of-the-art electronics and environmental control, the Alemnis system offers more versatility than any other.

Nanoindentation is a unique testing method used to assess various sample properties as a function of extremely small-scale surface deformation. Generally compact in design, nanoindentation systems can be utilized in a diverse set of applications for both organic and inorganic sample types. Biomaterials, ceramics, hard coatings, polymers, semiconductors, thin films, and more can routinely be analyzed using nanoindentation to determine numerous mechanical and dynamic characteristics.

In this blog post, Alemnis explores the properties that can be measured by nanoindentation.

Cracking and Fracture Toughness

Nanoindentation of brittle solid materials like ceramics often causes lateral cracks to form around the impression. These may extend across the surface or nucleate beneath it, each of which can contribute towards important end-product phenomena. Cracking, particularly nucleation, is primarily indicative of a material’s fracture toughness.

Fracture toughness defines a material’s resistance to catastrophic failure when pre-existing flaws propagate through the product. Determining the fracture toughness of a given sample requires an understanding of its Young’s modulus and the dimensions of the defect. These are used to determine the stress intensity factor; a critical parameter for measuring the force per unit of surface area required for voids and cracks to propagate and cause materials to fail. 

Nanoindentation measures Creep Deformation

Creep, or cold flow, is a measure of resistance to slow deformation in response to sustained mechanical stress. Nanoindentation can calculate the creep characteristics of a material by observing the increase in indenter tip depth as a function of time, as it is pressed into the sample surface at a constant force. This demands absolute precision and thermal stability of the nanoindentation system.

Nanoindentation detects Dislocation Nucleation

In materials science, dislocation nucleation occurs when the atomic bonds along a line in a lattice rupture due to shear forces. Homogenous dislocation nucleation requires many atomic bonds to break simultaneously which requires significant levels of shear stress. Nanoindentation is subsequently used to provide quantitative insights into material behavior at the onset of plasticity which is believed to contribute to dislocation, such as metal slipping or twinning.

Elastic Modulus and Hardness

Otherwise known as Young’s modulus, the elastic modulus of a material determines its resistance to deformation by compressive or bending forces. It is defined by the constant ratio of tensile stress to strain within the material’s elastic limits and can be used to calculate the applied tensile forces required for the material to plastically deform. The elastic modulus is intrinsically linked to both stiffness and hardness.

Nanoindentation was developed specifically for localized hardness testing of materials and has proven instrumental in testing their elastic moduli. As the elastic response of the surface sample is non-permanent, when the applied load of the nanoindenter tip is released, the sample surface will return to its original shape. Nanoindentation exceeds other hardness testing methodologies as it can also provide quantitative insight into elasticity.

Storage and Loss Moduli

Measuring the properties of viscoelastic samples with fixed geometries requires dynamic mechanical testing capable of acquiring storage and loss moduli. These refer to the measure of energy stored and lost in the elastic and viscous portions of the sample in response to linear stress. This is increasingly important for testing the properties of proprietary polymers and emerging biomaterials.

If you would like to learn more about the unique Almenis solution for nanoindentation testing, see our Product page

Nanoindentation with Alemnis

Alemnis specializes in small-scale mechanical testing for materials science research, offering a system equipped for both compression or tension modes. We can help you measure all the above properties and more.

Otherwise, contact us directly with any questions about our nanoindentation capabilities. 

Selected references

1. D. Tabor, Proc. R. Soc. A, 192 (1948) 247 (See article)
2. M. F. Doerner, D. S. Gardner and W. D. Nix, J. Mater. Res., 1 (1986) 845-851 (See article)
3. W. C. Oliver and G. M. Pharr, J. Mater. Res., 7 (1992) 1564-1583 (See article)
4. N. X. Randall, R. Christoph, S. Droz, C. Julia-Schmutz, Thin Solid Films, 290-291 (1996) 348-354 (See article)

Nanoindentation and micro-compression tests are widely used to interrogate the mechanical properties of materials using either load or displacement control via an indenter tip of known geometry. This instrumented indentation principle has evolved from conventional methods of testing hardness, such as the Brinell or Vickers techniques and has emerged as the ideal candidate technology for evaluating a wide range of sample properties, such as hardness, elastic modulus, fracture toughness, creep, stress relaxation, strain rate sensitivity and slip deformation. However, material characteristics vary significantly with changes in temperature.

In some cases, the temperature dependence of material characteristics may have little-to-no implications for downstream applications. The same cannot be said of materials intended for use in thermal processing, refractory applications or as cutting tool coatings operating at high speeds and temperatures. Such materials must be characterized in simulated in-service temperatures, which can be challenging for conventional nanoindentation equipment.

In this blog post, Alemnis explores which nanoindenter materials are most suited for high-temperature testing.

Properties of High-Temperature Indenter Materials

Many of the challenges and quirks associated with assessing the mechanical properties of materials at elevated temperatures were first explored and overcome by hot hardness testing. This underlines many of the unique principles of high-temperature indentation. While indenter probes were initially chosen based on the highest possible hardness and stiffness, hot hardness tests demonstrated the importance of characterizing how these properties might change under test conditions.

Chosen indenter materials must retain their hardness at test temperatures, which often vary from room temperature up to 600°C (1112°F) in many industrial applications. Materials with poor coefficients of thermal expansion that exhibit exceptional hardness at room temperature may soften under test conditions. This can dramatically change the indenter area function, which in turn will cause an error in the subsequently calculated hardness or elastic modulus by a proportional amount. In addition, the indenter tip may become permanently damaged and lose its sharpness. 

It is also important to assess mismatch in chemical reactivity between the indenter tip and the sample at high temperatures. Indenter materials that are inert at low temperatures may react with the sample material under test conditions, causing contamination of one or both materials in contact and, in extreme cases, dissolution of one material into the other.

Evaluating High-Temperature Indenter Materials

In order to perform a hardness test, the indenter material must be a minimum of 20% high hardness than the sample in order to generate plastic deformation. Ideally, the indenter should be > 100% harder in order to avoid rapid blunting or failure. A comprehensive analysis of high temperature indenter materials has been compiled by Wheeler and Michler (Ref. 1) and is shown in the hardness vs. temperature plot below. Although diamond is obviously one of the best indenter materials at ambient temperatures, its hardness decreases rapidly at elevated temperatures, although it still remains harder than all other candidates.

Diamond is the hardest engineering material available at all temperatures, from room temperature up to 1000°C (1832°F). It retains its exceptional intersection of mechanical properties throughout the most stringent high-temperature nanoindentation tests and remains the ideal solution for testing extremely hard materials like technical-grade ceramics and functional coatings. Cubic boron nitride (BN) is the next most noteworthy indenter material in terms of hardness at elevated temperatures, retaining its rigid structure up to operating temperatures of approximately 500°C (932°F) – after which it experiences an exponential drop in hardness.

Hot Vickers and Knoop hardness of indenter materials as a function of temperature with extrapolated Vickers hardness shown as dotted lines for materials where only Knoop hardness data were available (from Ref. 1)

While not as hard as cubic boron nitride, tungsten carbide (WC) is significantly better at retaining its hardness at high temperatures, with a maximum peak operating temperature of approximately 900°C (1652°F).

However, as mentioned, these material properties are merely one aspect of indenter performance. The risk of cross-contamination between the indenter tip, the sample, and oxygen molecules in the atmosphere is significantly increased at elevated temperatures. Oxide formation on indenter tips is practically unavoidable when heated to temperatures exceeding 400°C (752°F), for diamond, boron nitride, and sapphire (Al2O3). Tungsten carbide is one of the least reactive indenter materials at high temperatures, which enables its application for high-temperature nanoindentation testing of metal alloys.

In many ways, the material of the indenter tip must be diagnosed with some understanding of the material under test. While diamond and tungsten carbide represent the ideal frontrunners in terms of maintained mechanical properties and low reactivity, there are always exceptions that disprove the rule.

Read More: Guide to High Strain Rate Testing

Reactivity poses an additional issue in that it may impose a temperature limitation on many nanoindentation apparatuses, with an average maximum operating temperature in air of 200°C (392°F). This may be sufficient for proprietary tests, but it will not provide accurate insights into the material hardness of refractories and high-performance alloys. A typical solution to this is to conduct heated indentation tests in vacuum conditions.

High-Temperature Nanoindentation Testing with Alemnis

Alemnis offers a range of High Temperature (HT) modules for precision testing of samples with a heated nanoindentation tip up to 1000°C (1832°F). These retrofittable modules are compatible with all standard indenter tip materials which can be simply and easily exchanged between experiments.

If you would like to learn more about our high temperature modules for nanoindentation testing, please do not hesitate to contact Alemnis. 

1. J. M. Wheeler and J. Michler, Indenter materials for high temperature nanoindentation, Review of Scientific Instruments 84 (2013) 101301

How are Micropillars Produced?

Micropillar compression testing is a novel materials characterization technique that has emerged as a supplementary – and, in some instances, replacement – technology to indentation testing. Comparable to nanoindentation in terms of its ability to accurately quantify various mechanical properties of materials on the microscale, micropillar compression testing is a valuable addition to the technological arsenals of researchers concerned with myriad different sample types.

The underlying principles of these two techniques are similar, to the extent that near-identical instrumentation can be used to carry out both nanoindentation and micropillar compression tests. This has been demonstrated with materials as varied as metallic alloys, polymeric fibers, and refractory ceramics. The main advantage of micropillar compression over conventional nanoindentation is that the flat punch indenter maintains a constant contact area throughout the experiment so errors in area function are removed from the calculation of mechanical properties.

Example of a micropillar compression test on a micropillar built from a multilayer stack of different coatings

Outlining Micropillar Compression Testing

In a typical micro-compression test, a micrometer-scale (μm) pillar of sample material is compressed in a nanoindenter with a calibrated flat punch probe rather than a tip with a pyramidal geometry. Due to the extremely small dimensions of the micropillar, it is desirable to perform in situ positioning of the probe tip within the chamber of a scanning electron microscope (SEM). This eliminates errors associated with poor tip-optic calibration in ex situ testing conditions – yet it is not always appropriate to measure the deformation characteristics of materials in an SEM chamber. Certain polymers, for example, cannot be tested in the vacuum conditions of an SEM due to the potential for beam damage and cracking. In such instances, micropillar compression tests can be conducted in air under an optical microscope.

Each of these test methodologies can provide accurate insights into the real-world stress-strain properties of materials, including extremely complex behaviors like elastic moduli and microscale spatial variation of fracture toughness. Before these tests can be conducted, however, micropillars must be extracted from the bulk material.

Micropillar Fabrication

The primary method used to produce micropillars from bulk materials is focused ion beam (FIB) milling; a technology that mirrors the apparatus of a standard SEM array. Instead of a focussed beam of electrons, FIB setups utilize a high-energy gallium (Ga) ion beam to sputter the uppermost atomic layers of a material and facilitate ultra-precise sample machining on the sub-microscale. This process has become popularised in recent decades due to its outstanding accuracy in semiconductor etching applications.

Surface damage and implantation is a side-effect of FIB milling that could introduce measurement inaccuracies to micropillar compression tests. While gallium ion sources at high ion currents are desired for their superior top-down material removal rates, it may be necessary to ramp down the ion current during processing to optimize the structural and surface finish properties of the micropillar. This improves the yield of microscale samples with mechanical properties representative of the bulk material and decreases the likelihood of FIB induced damage that could affect compression results.

One of the disadvantages of FIB milling is that the micropillar in some materials may not have the same diameter over its whole length. An alternative is to fabricate micropillars from lithographic techniques where material is etched away after creating a surface mask. The advantage of lithographic techniques is that large arrays of micropillars can be fabricated in a few simple steps and much faster than by FIB milling. The shape of such pillars may also be more uniform. Some casting techniques are also available for producing micropillars, but then are based on the materials deposited rather than the bulk.

Examples of Si micropillar arrays (left) made by lithography and a single micropillar (right) just before contact with a diamond flat punch indenter.

Micropillar Testing with Alemnis 

Alemnis specializes in the development of value-added tools and solutions for advanced materials testing studies and applications. Our versatile portfolio of products includes the industry-leading Alemnis Standard Assembly (ASA); a compact and flexible nanoindenter suitable for both nanoindentation testing and micropillar compression experiments.

For further information about performing next-generation materials testing with the Alemnis Nanoindenter, please do not hesitate to contact Alemnis today.